I dont think mortarions really suits the character, as in my opinion it was less an ego to stroke and more self loathing and an inability to move past his own failures as an individual that drove him to just generally be a bitter spiteful person. Its why falling to nurgle actually kinda suited him in the end, as he was already a ball of misery and self loathing concealed behind a mask of stony bitter spite that all nurgle had to do was drive him to a point of utter despair and hopelessness and then pick up his new daemon primarch when mortarion was at his worse. It certainly helps to rationalize why he isn't out killing imperial scum as often as angron when he got daemonfied as I'm sure losing every important personal battle in your life would go a way to demoralizing any individual, even a primarch.
Buuut this is your poem and your interpretation of the primarchs so eh.
The only other thing i could mention would be that it doesnt feel like the poem really has a focus or overall goal in mind, might wanna work on that.